Add CONTRIBUTING.md #45

Summary

The README promises a CONTRIBUTING.md, but the repo has none.

What it should cover

• Prerequisites (.NET 10 SDK, see global.json) and dotnet build shelldocs.slnx / dotnet test shelldocs.slnx
• Running the example site (examples/ShellDocs.Preview) and checking a change in a static export (shelldocs build)
• The static-host rule for chrome: server-rendered state, data-* attributes and handlers in shelldocs.js, no @onclick for anything that must work without a Blazor runtime (see docs/ARCHITECTURE.md)
• Conventional Commits, one logical change per commit, each commit building and passing tests
• Adding a CHANGELOG entry under [Unreleased]

Then update the README's "Contributing" section to link it.